With the recent remarkable development of potable electronic equipment, communications equipment and the like, a strong demand for high energy density secondary batteries exists from the standpoints of economy and size and weight reductions. One prior art method for increasing the capacity of secondary batteries is to use oxides as the negative electrode material, for example, oxides of V, Si, B, Zr, Sn or the like or complex oxides thereof (see JP-A 5-174818 and JP-a 6-060867 corresponding to U.S. pat. no. 5,478,671), metal oxides quenched from the melt (JP-A 10-294112), silicon oxide (Japanese Patent No. 2,997,741 corresponding to U.S. pat. no. 5,935,711), and Si2N2O and GE2N2O (JP-A 11-102705 corresponding to U.S. pat. no. 6,066,414). Conventional methods of imparting conductivity to the negative electrode material include mechanical alloying of SiO with graphite, followed by carbonization (see JP-A 2000-243396 corresponding to U.S. pat. no. 6,638,662), coating of silicon particles with a carbon layer by chemical vapor deposition (JP-A 2000-215887 corresponding to U.S. pat. no. 6,383,686), and coating of silicon oxide particles with a carbon layer by chemical vapor deposition (JP-A 2002-042806).
The foregoing prior art methods are successful in increasing the charge/discharge capacity and energy density, but still leave several problems including insufficient cycle performance. They fail to fully meet the characteristics required in the market and are thus not necessarily satisfactory. A further improvement in energy density is desired.
In particular, Japanese Patent No. 2,997,741 uses silicon oxide as the negative electrode material in a lithium ion secondary cell to provide an electrode with a high capacity. As long as the present inventors have confirmed, there is left a room for further improvement as demonstrated by a still high irreversible capacity on the first charge/discharge cycle and cycle performance below the practical level. With respect to the technique of imparting conductivity to the negative electrode material, JP-A 2000-243396 suffers from the problem that solid-to-solid fusion fails to form a uniform carbon coating, resulting in insufficient conductivity. In the method of JP-A 2000-215887 which can form a uniform carbon coating, the negative electrode material based on silicon undergoes excessive expansion and contraction upon adsorption and desorption of lithium ions, meaning impractical operation, and loses cycle performance. Thus, the charge quantity must be limited. In JP-A 2002-042806, despite a discernible improvement of cycle performance, due to precipitation of silicon crystallites, insufficient structure of the carbon coating and insufficient fusion of the carbon coating to the substrate, the capacity gradually lowers as charge/discharge cycles are repeated, and suddenly drops after a certain number of charge/discharge cycles. This approach is thus insufficient for use in secondary cells. Even if this problem is overcome, the problem of low initial efficiency is left as long as the starting materials are silicon oxide-based materials.
The development of an electrode material having an increased charge/discharge capacity is very important and many engineers have been engaged in the research and development thereof. Under the circumstances, silicon and amorphous silicon oxides (SiOx) are of great interest as the negative electrode active material for lithium ion secondary cells because of their large capacity. Only few of them have been used in practice because of their shortcomings including substantial degradation upon repeated charge/discharge cycles, that is, poor cycle performance, and in particular, low initial efficiency. Making investigations from such a standpoint with the target of improving cycle performance and initial efficiency, the inventor found that CVD treatment of silicon oxide powder to provide a carbon coat led to a substantial improvement in performance as compared with the prior art. However, further improvements in long-term stability and initial efficiency are demanded.
In an experiment where CVD-treated silicon oxide was used as the negative electrode active material for lithium ion secondary cells, a rapid drop of charge/discharge capacity occurred after repetition of many charge/discharge cycles. From the structural aspect, the inventor investigated the cause of this problem. It was found that substantial volume changes occur upon occlusion and release of a large amount of lithium to cause particles to collapse, and the occlusion of lithium causes silicon or silicon compound, having low conductivity in the original state, to expand in volume so that the electrode itself lowers its conductivity. This results in a reduced current collecting capability, which prevents lithium ions from migrating within the electrode, causing drops of cycle performance and efficiency.
Continuing a study on the structure which not only has a stable surface conductivity, but is also stable against volume changes associated with occlusion and release of lithium, the inventor found that the above problems of lithium ion secondary cell negative electrode active material are overcome by coating surfaces of silicon crystallites or microparticulates with an inert robust substance such as silicon dioxide, and fusing carbon to part of surfaces of composite particles for imparting conductivity to the surfaces. The resulting material has a consistent high charge/discharge capacity and achieves drastic improvements in cyclic charge/discharge operation and efficiency thereof. Making investigations to develop a lithium ion secondary cell negative electrode active material having better cycle performance, the inventor found that a conductive silicon composite characterized by coating surfaces of particles of the structure having silicon crystallites dispersed in a silicon-based compound with carbon has good cycle performance and is thus effective for lithium ion secondary cell negative electrodes. That is, the conductive silicon composite is obtained by finely dispersing silicon crystallites and/or microparticulates in a silicon compound, typically silicon dioxide, and coating surfaces of the composite with carbon so as to achieve partial fusion, as described in JP-A 2004-47404 (U.S. ser. no. 10/246,426, Published Application no. 2003-215711; China Patent Application No. 02155814.0, Published Application No. 1513922). This composite material is improved in initial efficiency, but its initial efficiency is low as compared with the current carbon-based materials. Since the composite material had a satisfactory capacity and cycle performance, it was expected that its low initial efficiency could be cleared by various known approaches for increasing initial efficiency, for example, incorporating metallic lithium and/or organolithium compounds. Reference should be made to JP-A 11-86847, JP-A 2004-235057, and JP-A 2004-303597 for the addition of metallic lithium; and JP-A 5-226003 corresponding to U.S. pat. no. 5,316,875 and GS News Technical Report, Vol. 62-2, p. 63 (2003) for the addition of organic lithium.
However, in the actual process of manufacturing lithium ion secondary cells, the inclusion of the lithium addition step raises many problems. There remained a need for a negative electrode material which is improved in initial efficiency while maintaining the desired characteristics of conductive composite material.